Composite sandwich structures having resin matrix skins or face sheets adhered to a honeycomb or foam core are used extensively in aerospace, automotive, and marine applications for primary and secondary structure. The face sheets typically are organic matrix resin composites made from fiberglass, carbon, ceramic, or graphite reinforcing fibers and a thermosetting or thermoplastic matrix resin. Typically, the face sheets carry the applied loads, and the core transfers the load from one face sheet to the other or absorbs a portion of the applied load. In either case, it is important that all layers remain rigidly connected to one another. Noise suppression sandwich structure and sandwich structures for other applications are described in U.S. Pat. No. 5,445,861, which I incorporate by reference.
Keeping the face sheets adhered to the foam core is problematic. With reference to a foam core sandwich panel, the most common source of face sheet separation stems from the relatively weak adhesive bond between the face sheets and the foam core. That is, the pulloff strength of the face sheets in shear is low. Efforts to strengthen the bond have generally focused on improving the adhesive, but those efforts have had limited success.
Differences in the coefficient of thermal expansion (CTE) of the different material layers can also give rise to separation or delamination. As a result, as temperatures oscillate, the foam or face sheet may expand or contract more quickly than the adjoining layer. In addition to causing layer separation, CTE differences can significantly distort the shape of a structure, making it difficult to maintain overall dimensional stability.
Conventional sandwich structure optimizes the thickness of a structure to meet the weight and/or space limitations of its proposed application. Sandwich structures are desirable because they are usually lighter than solid metal or composite counterparts, but they may be undesirable if they must be larger or thicker to achieve the same structural performance. Providing pass-throughs (i.e. holes), which is relatively easy in a solid metal structure simply by cutting holes in the desired locations, is more difficult in a sandwich structure because holes may significantly reduce the load carrying capability of the overall structure.
Foster-Miller has been active in basic Z-pin research. U.S. Pat. No. 5,186,776 describes a technique for placing Z-pins in composite laminates. The matrix resin is heated and softened by ultrasonic energy transmitted through the ultrasonic needle, which, then, penetrate the laminate, moving the laminate fibers aside. The needle is withdrawn and a pin inserted or the pin is fed through the needle prior to its removal, in either case, thereby, inserting a Z-direction reinforcing fiber into the laminate. Cooling yields the pin reinforced composite. U.S. Pat. No. 4,808,461 describes a structure for localized reinforcement of composite structure. A body of thermally decomposable material that has substantially opposed surfaces is placed on the composite structure. A plurality of reinforcing elements (pins) captured in the body extend generally perpendicular to one body surface. A pressure plate (i.e., a caul plate) on the other opposed body surface drives the pins into the composite structure as the body is heated under pressure and decomposes. I incorporate U.S. Pat. Nos. 4,808,461 and 5,186,776 by reference.
A need exists for a method of forming a sandwich structure that (1) resists distortion and separation between layers, in particular, separation of the face sheets from the core; (2) maintains high structural integrity; (3) resists crack propagation; and (4) easily accommodates the removal of portions of foam core, as required by specific applications. The method should allow the structure to be easily manufactured and formed into a variety of shapes. In U.S. patent application Ser. No. 08/582,297 entitled "Pin-Reinforced Sandwich Structure," which I incorporate by reference, I described a method of forming a pin-reinforced foam core sandwich structure. I positioned first and second face sheets of uncured fiber-reinforced resin (i.e., prepreg or B-stage thermoset) on opposites side of a foam core. The core had at least one compressible sublayer and a plurality of Z-pins spanning the foam between the face sheets. I inserted the Z-pins into the face sheets during autoclave curing of the face sheet resin much like Foster-Miller, but without decomposing the foam. During autoclave curing, the pressure crushed the compressible sublayer and drove the Z-pins into one or both of the face sheets to form the pin-reinforced foam core sandwich structure. By removing at least some of the foam core by dissolving, eroding, melting, drilling, or the like to leave a gap between the face sheets, I converted the foam core sandwich structure to column core material.
As I described, the foam core generally includes a high density foam sublayer, and at least one low density foam sublayer that is crushable under the autoclave pressure. The preferred arrangement includes a first and second low density foam sublayer, one placed on each side of the high density sublayer. The plurality of Z-pins are placed throughout the foam core in a regular array normal to the surface or slightly off-normal at an areal density of about 0.375 to 1.50% or higher, as appropriate, extending from the outer surface of the first low density foam sublayer through to the outer surface of the second low density foam sublayer. Expressed another way concerning the arrangement of the pins, I might use 40-50 pin/in.sup.2 or more. Preferably, the foam sublayers are formed of polyimide or polystyrene, the Z-pins are formed of stainless steel or graphite, and the face sheets are formed of partially cured thermosetting fiber/resin or thermoplastic composite materials.
Rorabaugh and Falcone suggested increasing pulloff strength by ordering of the Z-pins into regular ordered structural configurations or with resin fillets, as described in copending U.S. Pat. No. 5,869,165 "Highly Ordered Z-Pin Structures," which I incorporate by reference. They form resin fillets around the fiber/resin interfaces at the contact faces of the foam core by dimpling the foam to create a fillet pocket, or they arrange the pins in an ordered fashion such as a tetrahedral or a hat section configuration. The ordered pins provide a tie between the two skins and miniature structural support suited better for load transfer than normal or random off-normal (interlaced) or less ordered pin configurations.
In U.S. Pat. No. 5,589,016, Hoopingarner et al. describe a honeycomb core composite sandwich panel having a surrounding border element (i.e., a "closeout") made of rigid foam board. The two planar faces of the rigid foam board are embossed or scored with a pattern of indentations usually in the form of interlinked equilateral triangles. The indentations are sufficiently deep and sufficient in number to provide escape paths for volatiles generated inside the panel during curing and bonding of the resin in the face sheets to the honeycomb core and peripheral foam. The scoring prevents the development of excessive pressure between the face sheets in the honeycomb core that otherwise would interfere with the bonding. I incorporate this application by reference.
Fastening composite structure introduces stress concentrations that can pose problems, especially the formation of cracks in the "zero" plies where the resin is asked to hold the fibers together and to transfer load from the fastener to the fibers. I have learned that off-normal Z-pins reduce the likelihood of cracking in these zero plies in fastened composites or composite sandwich structure.